A rack-and-pinion type steering apparatus is generally comprised of a housing, a pinion rotatably supported by this housing, a rack bar on which rack teeth meshing with this pinion are formed, a rack guide disposed in a housing to slidably support the rack bar, and a spring for pressing this rack guide toward the rack bar.
In such a rack-and-pinion type steering apparatus, an iron-based sintered metal or a synthetic resin is used for the rack guide for slidably supporting the rack bar. However, in the case of the rack guide formed of an iron-based sintered metal, although it has sufficient mechanical strength against an impact load from the rack bar, the efficiency of a steering system declines since the frictional resistance to sliding is large, so that a problem remains in controllability. Meanwhile, as for the rack guide formed of a simple substance of synthetic resin, although it is conversely capable of reducing the frictional resistance to sliding, the following problems are encountered among others: It is inferior in mechanical strength against an impact load. It undergoes variations in dimensions due to such as die shrinkage, and it is difficult to effect molding with high dimensional accuracy and maintain the dimensional accuracy after molding. Furthermore, after it is built into the housing, it is subjected to the effect of the temperature rise of the steering apparatus and undergoes thermal expansion and shrinkage, which causes thermal deformation and creep, making it difficult to slidingly support the rack bar smoothly.
In the rack guide formed of a simple substance of synthetic resin, techniques have been proposed in which a reinforcing material constituted by a core bar is insert molded or embedded in a main body made of a synthetic resin particularly for the purpose of improving the mechanical strength against an impact load (disclosed in JP-UM-B-59-8932 and JP-UM-A-58-139275). However, with these techniques as well, the problem of the dimensional accuracy attributable to the die shrinkage of the synthetic resin and problems such as the thermal expansion, shrinkage, thermal deformation, and creep (plastic flow) of the synthetic resin ascribable to the temperature rise of the steering apparatus still remain unresolved.
As rack guides which overcome the above-described problems, rack guides have been proposed in which a slider made of a synthetic resin having self-lubricating properties and abrasion resistance on its sliding surface side for slidingly supporting the rack bar, or a slider made of a multilayered material of a three-layered structure consisting of a steel sheet, a metal sintered layer deposited integrally on this steel sheet, and a synthetic resin layer impregnating and coated on this sintered layer, is combined with a rack guide base body formed of a metal such as a sintered metal or a synthetic resin containing a reinforcing filler (disclosed in JP-A-50-102027, JP-UM-B-1-27495 and JP-UM-B-2-46452).
In comparison with the rack guide mentioned before, the above-described rack guide formed by the combination of the slider and the rack guide base body has advantages in that the mechanical strength is enhanced, and that the frictional resistance to sliding is reduced.
In the rack guide formed by the combination of the slider and the rack guide base body having the above-described advantages, particularly in a case where aluminum or an aluminum alloy is used for the rack guide base body, a problem in its manufacture has been presented. Namely, in the case where the rack guide base body is fabricated from aluminum or an aluminum alloy, the die casting process is generally adopted. According to this die casting process, molten metal of aluminum or an aluminum alloy is passed through a runner channel from a gate and is forced into a cavity formed in the shape of the rack guide base body. At that time, however, the formation of runners at the junction between the runner channel and the cavity and the junction between the cavity and an overflow well is made inevitable. Then, as these runners are cut off, the rack guide base body is formed.
Incidentally, since the gap between the rack guide with a circular cross section and the cylindrical housing of the rack-and-pinion type steering apparatus in which the rack guide is disposed is set to be 1 to 20 μm, even after the aforementioned runners are cut off, the presence of projecting portions as cut remnants of the gate is made inevitable on the outer peripheral surface of the rack guide base body. Accordingly, with the rack guide base body on the outer peripheral surface of which the projecting portions remain, there is a problem in that unless the entire outer peripheral surface of the base body is subjected to machining, the rack guide cannot be disposed in the cylindrical housing. This constitutes a factor leading to higher cost due to the machining of the entire surface. The same holds true of a case where zinc or a zinc alloy is used as the molten metal.